It might be a good idea to do a short search first, or even a brief scan of the current topics. This same question has come up twice this week already.
Read here for suggestions:
https://www.mobileread.com/forums/sho...d.php?t=119731
Note if you aren't getting chapters when editing with Sigil it's because you Chapter headings aren't surrounded by <hx> tags.
You can try using Heuristic Processing under conversion to detect and add proper mark up for chapters.